Hydraulic control system for automatic transmission

ABSTRACT

The rotational speed of an engine Ne and a drum rotational speed Nt of a clutch Cl are compared against each other to determine a power on/off. For a 2→3 and a 2→4 shift in the power off condition, a timing solenoid SL5 is turned on (duty cycle of 100%) to accelerate the termination of a preceding speed range, thus increasing a time delay between such termination and the activation of a new speed range to be established. For other shifts, a time difference between the termination of a preceding speed range and the activation of a new speed range to be established is adjusted, by energizing the solenoid SL5 in accordance with the throttle opening θ and the vehicle speed No such that SL5 is energized with a high duty cycle for a longer θ and with a low duty cycle for a smaller θ.

FIELD OF THE INVENTION

The invention relates to an automatic transmission in which a torquefrom the output shaft of an engine is transmitted to a load drivingshaft and in which a gear ratio or the ratio of the rotational speed ofthe output shaft of the engine with respect to the rotational speed ofthe load driving shaft is automatically changed, and in particular, to apressure control of brakes and clutches used in the transmission duringa shift operation in order to suppress shift shocks.

PRIOR ART

In an automatic transmission of the kind described, a shift from a speedrange to another takes place by removing an oil pressure from at leastone of brakes and clutches used in the automatic transmission whilesupplying an oil pressure to at least another one. There is a likelihoodthat shift shocks may result from such a switching of the oil pressures.To accommodate for this, in the prior art practice, accumulators areconnected to the brakes and clutches used in the automatic transmission,and the back pressures on the accumulators are regulated so as toprevent the occurrence of shocks during a shift operation (see JapaneseLaid-Open Patent Application No. 38,553/1981). However, the regulationis performed by using a mechanical valve, which results in a coarsepressure regulation. It is desirable to provide a more smooth andappropriate pressure regulation. At this end, Japanese Laid-Open PatentApplication No. 149,657/1986 discloses an automatic transmission inwhich a hydraulic circuit includes an electrically energized pressurecontrol valve for regulating the back pressure on accumulators. The dutycycle with which the pressure control valve is electrically energized iscontrolled in accordance with one of the opening of a throttle valve, ashift mode, an oil temperature of an engine, the temperature of anengine cooling water, the temperature of air suction to an engine, anoil temperature of the automatic transmission, a particular position ofa pattern select switch, an engine torque, the rotational speed of anengine, a supercharger pressure of an engine, a fuel injection of anengine, an output torque of the automatic transmission and therotational speed of the output shaft of the automatic transmission. Inthis manner, the back pressure of an accumulator is regulated bycontrolling the duty cycle of the pressure control valve. This allows afine regulation of the back pressure on accumulators because theenergization level or the duty cycle of the pressure control valve canbe electrically controlled, thus allowing a more appropriate backpressure to be established on the accumulator and smoothly regulated.

During a shift operation, a gear ratio of the automatic transmission ischanged by terminating a speed range which prevailed before the shiftoperation occurs and activating a speed range which is to beestablished. However, when the timing between the termination andactivation is properly matched, shift shocks or a blow-up of an enginemay result.

SUMMARY OF THE INVENTION

It is an object of the invention to achieve a more fine adjustment ofthe timing to yield a better result in suppressing the occurrence ofshift shocks or an engine blow-up.

The invention relates to a hydraulic control system for automatictransmission including a hydraulic circuit (FIG. 2) for selectivelysupplying an oil pressure to or selectively removing an oil pressurefrom brakes and clutches used in an automatic transmission which isdisposed between an output shaft of an engine and a load driving shaft,accumulators in the hydraulic circuit and connected to the brakes andclutches, a pressure control valve (SL6) for regulating a back pressureon the accumulator, and shift control means (130) for determining ifthere is a need to effect a shift operation and for performing a shiftoperation whenever such need is found.

In accordance with the invention, the hydraulic control system comprisesan electrically energized timing control valve (SL5) for providing atime delay between the termination of the transmission of a torque in aspeed range which prevails before a shift operation to the initiation ofthe transmission of a torque in a new speed range which is to beestablished; opening detecting means (138) for detecting the opening (θ)of a throttle valve of the engine; speed detecting means (142) fordetecting the rotational speed (No) of the output shaft (39) of theautomatic transmission; and delay control means (130) operable whenevershift control means (130) operates to perform a shift operation todetermine an energization level (duty cycle) which results in a longertime delay for a greater throttle valve opening (θ) and a shorter timedelay for a higher rotational speed (No) and to energize the timingcontrol valve (SL5) in accordance with such energization level, andsubsequently at a given time interval (0.4 sec) thereafter, to determinean energization level (duty cycle) in accordance with the throttle valveopening (θ) and the rotational speed (No) which again results in alonger delay time for a higher throttle valve opening (θ) and a shorterdelay time for a higher rotational speed (No) and to energize the timingcontrol valve (SL5) in accordance with such energization level.

It is to be noted that reference numerals and characters appearing inparentheses refer to corresponding elements or parts shown in anembodiment to be described later.

It will be noted that the throttle valve opening (θ) and the rotationalspeed (No) are parameters representing the running load as a vehicle isbeing propelled. The opening (θ) is substantially directly proportionalto the running load while the rotational speed (No) is inverselyproportional to the running load. A shift operation which takes placeunder a high running load is likely to cause shocks. In accordance withthe invention, a delay time between the termination of a speed rangewhich prevails before a shift operation to the activation of a new speedrange to be established is increased substantially in direct proportionto the running load by means of the timing control means (SL5), therebysuppressing the occurrence of shift shocks.

Although the vehicle speed changes at a relatively low rate of change,the throttle valve opening (θ) may change at a relatively high rate.Accordingly, there may be a large change during a shift operation. Inaccordance with the invention, a delay time is determiend by the timingcontrol valve (SL5) in accordance with the opening (θ) and therotational speed (No) at the commencement of the shift operation, andsubsequently the delay time is updated again in accordance with theopening (θ) and the rotational speed (No). Hence, the delay time can beappropriately modified in accordance with any change in the opening (θ)and the rotational speed (No) during the shift operation, furtherreducing the probability that shift shocks may result.

Other objects and features of the invention will become apparent fromthe following description of an embodiment thereof shown in thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a transmission mechanism according to oneembodiment of the invention;

FIGS. 2a and 2b are block diagrams, each showing one-half of a hydrauliccircuit which supplies an oil pressure to or removes an oil pressurefrom various brakes and clutches located within the transmissionmechanism shown in FIG. 1, FIGS. 2a and 2b being joined together alongIIA--IIA and IIB--IIB lines shown to form one hydraulic circuit;

FIG. 3 is a table representing the relationship between the combinationsof energization and deenergization of solenoid valves (SL1 to SL3) usedin the hydraulic circuit shown in FIGS. 2a and 2b to determine the speedrange and the engagement and disengagement of various brakes andclutches shown in FIG. 1;

FIG. 4 is a block diagram of an electrical circuit for energizingsolenoid valves (SL1 to SL3) shown in FIGS. 2a and 2b, a solenoid valve(SL4) for controlling a lockup, a timing solenoid valve (SL5) and alinear solenoid valve (SL6);

FIGS. 5a, 5b, 5c, 5d and 5e are flow charts representing the controloperations (main routines) by a microcomputer of a control board 130shown in FIG. 4;

FIGS. 6a and 6b are flow charts indicating the detail of "detection ofTSO, TSE" (64) shown in FIG. 5d;

FIGS. 7a and 7b are flow charts indicating the detail of "control oflinear solenoid" (65) shown in FIG. 5d;

FIGS. 8a, 8b and 8c are flow charts indicating the detail of "control oftiming solenoid" (66) shown in FIG. 5d;

FIG. 9a is a timing chart indicating the timing of the determination ofa shift operation, a shift output and the completion of a shiftoperation which are executed by the microcomputer in the control board130 shown in FIG. 4;

FIG. 9b graphically shows an oil pressure which is output from thelinear solenoid valve SL6 against the current level which is used toenergize the coil thereof;

FIG. 9c graphically shows the current level which is used to energizethe linear solenoid valve SL6 and which is determined by themicrocomputer in the control board 130 shown in FIG. 4 on the basis of athrottle valve opening θ and the rotational speed Nt of a drum in aclutch C₁ ;

FIG. 10a graphically shows a correction factor K11 which is dependent onan oil temperature and which is used in the determination of the dutycycle shown in FIG. 7 (at 117 to 128);

FIG. 10b graphically shows another correction factor K12, which is usedin the determination of the duty cycle shown in FIG. 7 (at 117 to 128),as a function of the opening θ of the throttle valve;

FIG. 10c graphically shows a correction factor K2, which is used in thedetermination of the duty cycle shown in FIG. 7 (at 117 to 128), as afunction of the shift mode and the opening θ of the throttle valve;

FIG. 10d graphically shows a relationship between a time reference valueTS for a mechanical shift corresponding to the opening θ of the throttlevalve and corresponding regions in which such reference value is used;

FIG. 11a graphically shows a change in the oil pressure of a brake B₀and a clutch C₂ shown in FIG. 1 during a 2→3 shift solid linerepresenting a power on shift operation;

FIG. 11b is a graphical representation of regions divided in accordancewith the opening θ of the throttle valve and the rotational speed N₀ ofa wheel driving shaft, the duty cycle for the timing solenoid SL5 shownin FIG. 2b being determined in a manner corresponding to such region;

FIG. 11c is a timing chart showing the timing when the duty cycle of thetiming solenoid valve SL5 shown in FIG. 2b is changed during a power onshift from a third or a fourth speed range to a second speed range;

FIG. 11d graphically shows an actual example of the region segmentationshown in FIG. 11b;

FIG. 11e graphically shows the relationship between the regions shown inFIG. 11d and a duty cycle which is determined in a corresponding manner;

FIG. 11f graphically shows a rising response in the oil pressure of abrake B₁ shown in FIG. 1 which results from the duty cycle determined inthe manner illustrated in FIG. 11e during a 1→2 shift;

FIG. 11g graphically shows an oil pressure rising response of a brake B₂during a 2→1 shift;

FIGS. 12a, 12b, 12c, 12d, 12e, 12f and 12g are a series of timing chartsindicating the timings for turning on and off of the speed rangeestablishing solenoid valve SL3 and the timing solenoid valve SL5 shownin FIGS. 2a and 2b during a shift operation from 1.5 speed range toanother as well as during a reverse shift operation;

FIG. 13a is a table of data which is made reference to when determininga change-over point X between on/off condition of the speed rangeestablishing solenoid valve SL3 shown in FIG. 12b;

FIG. 13 b graphically shows a change in the pressures of the brakes B₀to B₂ shown in FIG. 1 during a 1.5→2 shift operation shown in FIG. 12b;

FIG. 13c is a table of variance values to be added to the rotationalspeed of the drum of the clutch C₁ in a manner corresponding to therotational speed N₀ of the output shaft in order to substantiallyeliminate any resulting shock during a 1.5→2 shift operation shown inFIG. 12b;

FIG. 14a is a timing chart indicating a change in the duty cycle of thesolenoid valve SL4 as a time sequence when a directly coupled clutch 50shown in FIG. 1 is energized (or locked up);

FIG. 14b graphically shows the duty cycle of the solenoid valve SL4which is determined according to the opening θ of the throttle valve andwhich is to be assigned to Dt1 shown in FIG. 14a;

FIG. 14c is a timing chart indicating a regulation applied to the dutycycle of the solenoid valve SL4 in order to reduce resulting shocksduring a shift operation in which the directly coupled clutch 50 shownin FIG. 1 is energized;

FIG. 14d graphically shows the duty cycle of the solenoid valve SL4which is determined corresponding to the opening θ of the throttle valveand which is to be assigned to Dt2 shown in FIG. 14c;

FIG. 15a graphically shows a family of reference vehicle speeds used indetermining the need to perform a shift operation when an economy modeis not specified during the "decision for any shift operation" (14)shown in FIG. 5a; and

FIG. 15b graphically shows a family of reference vehicle speeds whichare used in determining the need to perform a shift operation when aneconomy mode is specified during the "decision for any shift operation"(14) shown in FIG. 5a.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a general mechanism according to one embodiment of theinvention. An automatic transmission shown in FIG. 1 comprises a torqueconverter 1 with a directly coupled clutch 50, a subtransmission 2formed by an overdrive mechanism, and a main transmission 3 formed by achange gearing and having three forward speed ranges and one reverserange, which are coupled together in the sequence named above.

The torque converter 1 comprises a pump 5, a turbine 6 and a stator 7which are assembled together in a manner known in itself. The pump 5 isconnected to a crankshaft 8 of an engine while the turbine 6 isconnected to a turbine shaft 9, which represents an output shaft of thetorque converter 1 and also forms an input shaft to the subtransmission2 where it is connected to a carrier 10 of a planetary gearing thereof.A directly coupled clutch 50 is disposed between the crankshaft 8 of theengine and the turbine shaft 9. When operated, the clutch 50mechanically couples the crankshaft 8 and the turbine shaft 9 together(lockup).

A planetary pinion 14 is rotatably supported by the carrier 10 andmeshes with a sun gear 11 and a ring gear 15. Disposed between the sungear 11 and the carrier 10 are an overdrive multi-disc clutch C₀ and anoverdrive one-way clutch F₀, and an overdrive multi-disc brake B₀ isdisposed between the sun gear 11 and a housing or an overdrive casingwhich houses the overdrive mechanism.

The ring gear 15 of the subtransmission 2 is connected to an input shaft23 of the main transmission 3. A front multi-disc clutch C₁ is disposedbetween the input shaft 23 and an intermediate shaft 29, and a reversemulti-disc clutch C₂ is disposed between the input shaft 23 and a sungear shaft 30. A mutli-disc brake B₁ is disposed between the sun gearshaft 30 and the casing of the transmission, and a sun gear 32 mountedon the sun gear shaft 30 forms two trains of planetary gearing togetherwith a carrier 33, a planetary pinion 34 which is carried by thecarrier, a ring gear 35 which meshes with the pinion, another carrier36, a planetary pinion 37 which is carried by the carrier 36, and a ringgear 38 which meshes with the pinion 37. A one-way clutch F₁ and a brakeB₂ are interposed between the carrier 36 and the casing of thetransmission. The ring gear 35 which is disposed in one of the twotrains of the planetary gearing is connected to the intermediate shaft29. The carrier 33 of this planetary gearing is connected to the ringgear 38 of the other planetary gearing, and the carrier and the ringgear are connected to the output shaft 39. In response to an output froman engine and in accordance with the vehicle speed, an automatictransmission with an overdrive unit as mentioned above is controlled bya hydraulic control system to be described later to engage or releasevarious clutches and brakes in order to perform a shift operation tofour forward ranges (a first, a second, a third, a fourth speed range:O/D) including the overdrive (O/D), one forward range by a manual shiftoperation (1.5 speed range=LS) and one reverse range.

A hydraulic circuit which selectively activates the clutches C₀, C₁, C₂and brakes B₀, B₁, B₂ and the directly coupled clutch 50 of the torqueconverter in the automatic transmission to perform an automatic shiftoperation is shown in FIGS. 2a and 2b.

The hydraulic circuits shown in FIGS. 2a and 2b include an oil sump 100,an oil pump 101, a pressure regulating valve 102, an auxiliary pressureregulating valve 103, a linear solenoid SL6, a manual valve 200, 1-2shift valve 220, 2-3 shift valve 230, 3-4 shift valve 240, a 1/2 shiftcontrolling solenoid valve SL1, a 2/3 shift controlling solenoid valveSL2, a 3/4 shift controlling solenoid valve SL3, accumulators 260, 270,280, 290, an accumulator pressure control valve 110, a modulator valve90, an orifice control valve 80, a timing solenoid valve SL5, a lowcoast modulator valve 250, a discharge pressure valve 70, a lockupcontrol valve 360, a lockup control signal valve 370, a lockupcontrolling solenoid valve SL4 and other hydraulic elements and oilpaths which provide connections between these valves and which provide aservo circuit connection for the oil pressures of the clutches andbrakes.

A working oil which is pumped from the sump 100 by the pump 101 isregulated to a given oil pressure (line pressure) by means of thepressure regulating valve 102, and is then fed to oil paths 104 and 105.The oil pressure which is supplied through the oil path 105 to theauxiliary pressure regulator valve 103 is then regulated by the linearsolenoid valve SL6 to provide a torque converter pressure, a lubricantpressure and a cooler pressure which are determined in accordance with athrottle opening and a vehicle speed. The manual valve 200 which isconnected to the oil path 104 is mechanically coupled to a shift leverwhich is mounted adjacent to a driver's seat, and may be displaced to P,R, N, D, S and L positions corresponding to the range of the shift leverthrough a manual operation.

In the hydraulic circuit shown in FIG. 2, a combination of theenergization or deenergization of the shift controlling solenoid valvesSL1 to SL3 determines a particular speed range of the automatictransmission shown in FIG. 1 (figures appearing in the columns of D, S,L) as indicated in the table of FIG. 3 where o represents theenergization and x represents the deenergization of the respectivesolenoids. It is to be noted that O/D appearing in the column of speedrange in FIG. 3 represents the fourth speed range (overdrive) while LSrepresents a speed range intermediate the first speed range (having agear ratio of 2.905) and a second speed range (having a gear ratio of1.530), and which can be denoted as a 1.5 speed range having a gearratio of 2.257.

Directing our attention to features of the described automatictransmission which are shown in FIGS. 1, 2a and 2b, it is to be notedthat LS (1.5 speed range) is located between the first and the secondspeed range. The purpose of LS (1.5 speed range) is to realize a smoothrunning response which assures a relatively high speed with a sufficientpower when running on a up-grade road and to realize a smooth runningresponse which assures an appropriate engine braking operation whilerunning down a down-grade road even though the vehicle with a heavy loadis running an up-grade road, where the choice of the first speed rangewill result in a retarded speed and the choice of a second speed rangewill result an insufficient power to cause an unstable running condition(there will be a relatively frequent shift between the first and thesecond speed range, and such shift operation is likely to cause shocks)and even though the choice of the first speed range will result in anexcessive engine braking operation and the choice of the second speedrange will be a poor engine braking operation when the vehicle isrunning down a down-grade road (there will be a relatively frequentshift between the first and the second speed range, and such shiftoperation is also likely to cause shocks).

If an arrangement is made to establish the 1.5 speed range (LS)automatically between the first and the second speed range of D range,the speed zones contained in the first and the second speed range of Drange will be divided into three segments rather than two segments,resulting in a reduced width of the resulting speed zones, causing anincrease in the frequency of the shift operation and causing an unusualsensation of a driver, which is caused by resulting shocks and achangeover between speed ranges.

For this reason, in the present embodiment, the 1.5 speed range (LS) isestablished in response to a command for the 1.5 speed range (LS) issuedby a switch operation by a driver when the shift lever assumes aposition within L range.

In the automatic transmission shown in FIG. 1, if the subtransmission 2assumes an overdrive condition (O/D; C₀ turned off, B₀ released) and themain transmission 3 assumes a first speed range (C₁ turned on, C₂ turnedoff, B₁ released and B₂ engaged), the resulting gear ratio will assume avalue (2.257) which is intermediate the gear ratio (2.950 and 1.530) ofthe first and the second speed range, and accordingly this value isdifined to be 1.5 speed range (LS). When commanding the 1.5 speed range,the subtransmission 2 is established in its O/D condition while the maintransmission 3 is chosen to be in its first speed range (FIG. 3).

Another feature of the automatic transmission shown in FIGS. 1, 2a and2b relates to the provision of accumulators 260, 270, 280 and 290 whichserve preventing the occurrence of shocks as a result of the shiftoperation. Each accumulator internally contains a piston which is urgedupward by a coiled compression spring, not shown. The back pressure onthis piston is controlled by the linear solenoid valve SL6 through theaccumulator pressure control valve 110, while the timing solenoid valveSL5 is effective to provide a smooth changeover of pressures as theassociated clutches and brakes are activated or terminated, by supplyingan oil pressure to the brake B₁ through both orifices 81 and 82 (aretarded rising) or by bypassing through the orifice 82 (a rapidrising).

In this embodiment, an output pressure from the linear solenoid valveSL6 (a pressure which is proportional thereto is supplied by theactuator pressure control valve 110 to each of the accumulators 260 to290) is substantially inversely proportional to the duty cycle of thecurrent flow which energizes the valve SL6. Specifically, as the dutycycle of the current flow which energizes the linear solenoid valve SL6increases, the back pressure on the piston of each of the accumulators260 to 290 reduces, thus reducing the oil pressure which causes theengagement of the clutch or brake associated therewith. In a particularshift mode which is likely to cause shocks as a result of a shiftoperation, such shocks are prevented by establishing a high value ofduty cycle for the linear solenoid valve SL6.

When the timing solenoid valve SL5 is turned on to establish the dutycycle of 100%, it connects a pilot pressure chamber of the orificecontrol valve 80 to a drain, whereby the orifice control valve 80bypasses an oil path connecting between the orifices 81, 82 to the brakeB₁ (accumulator 280). As a result, the pressure will be reduced at arapid rate when releasing the brake B₁, whereby the brake will bereleased rapidly (or conversely, when engaging the brake, the engagementwill occur rapidly). However, when the timing solenoid valve SL5 isturned off to establish the duty cycle of 0%, the oil pressure in thepilot pressure chamber of the orifice control valve 80 rises, wherebythe orifice control valve 80 disconnects the oil path connecting betweenthe orifices 81 and 82 from the brake B₁ (accumulator 280). Accordingly,the brake B₁ will be connected to the oil pressure line through theorifices 81 and 82, so that the pressure will be reduced at a retardedrate when releasing the brake B₁ (the brake will be released slowly)while the pressure rises at a slow rate when engaging the brake B₁ (thebrake will be engaged slowly).

FIG. 4 schematically shows an electrical control system which controlsthe energization of the shift solenoid valves SL1 to SL3, the lockupcontrol solenoid valve SL4, the timing controlling solenoid valve SL5and the linear solenoid valve SL6 of the hydraulic circuit shown inFIGS. 2a and 2b.

The heart of the electrical control system comprises a control board130, which comprises a microcomputer (hereafter referred to as CPU) andan input/output interface assembled together on a printed circuit board.Connected to the input interface of the control board 130 are a pulsegenerator 140 which generates electric pulses at a frequency which isproportional to a rotational speed Ne of the input shaft 8 of the torqueconverter 1 (or the output shaft of the engine), a pulse generator 141which generates electric pulses at a frequency which is proportional toa rotational speed Nt of the output shaft 23 (C₁ drum) of thesubtransmission 2, a pulse generator 142 which generates electric pulsesat a frequency which is proportional to a rotational speed No of theoutput shaft (a wheel driving shaft) 39 of the main transmission 3, afirst speed range (LS) command switch 131, a fuel saved running commandswitch 132, a fourth speed range (O/D) inhibit command switch 133, ashift lever position detecting switch 134, a sensor 135 for detectingthe temperature of an engine cooling water, a sensor 136 for detectingthe temperature of an oil in the oil sump 100 in the hydraulic circuitshown in FIGS. 2a and 2b, a brake switch 137, a throttle opening sensor138, an idling opening sensor 139, an ignition key switch IGS andonboard batteries 163, 164.

Various input switches mentioned above are connected to associatedindicator lamps. Specifically, when the first speed range command switch131 is closed, commanding the selection of the first speed range, a lamp152 will be illuminated. When the fuel saved running command switch 132is closed, providing a command to run the vehicle in a fuel saving mode,a lamp 153 will be illuminated. When the fourth speed range (O/D)inhibit switch 133 is closed, commanding a corresponding inhibit, a lamp154 will be illuminated.

When the shift lever is in its N (neutral) position N, the switch 134will be in its D position and a lamp 155 will be illuminated. In R(reverse) position of the shift lever, a lamp 156 will be illuminated.In P (parking) position of the shift lever, a lamp 157 will beilluminated. In the L position of the shift lever, the switch 134assumes its L position and lamps 158 and 161 will be illuminated. In theS position of the shift lever, lamps 159, 161 will be illuminated. Inthe D position, lamps 160 and 161 will be illuminated. The illuminationof the lamp 161 indicates that the shift lever assumes a forwardposition. When the brake switch 137 is closed, indicating that a brakepedal has been depressed, a lamp 162 will be illuminated.

Electric pulses developed by the pulse generators 140, 141 and 142 areshaped into rectangular waves of a given level by the input interfacebefore they are applied to an external interrupt input port of CPU inthe control board 130. A binary signal (assuming a high level H equal nocommand when the associated switch is open and indicating a low level Lequal a command condition when closed) such as from the first speedrange command switch 131, the shift lever position detecting switch 134or the like is fed to the input interface where a chattering occurringbetween a change between its levels are removed to provide a singlerising or falling pulse as it is applied to the input port of CPU in thecontrol board 130. Analog signals from the water temperature sensor 135,the oil temperature sensor 236 and the throttle valve opening sensor 138are subject to a smoothing and level converting amplification in theinput interface before they are applied to an analog signal input port(A/D conversion input port) of CPU in the control board 130.

In response to the falling edge of a pulse developed by any of the pulsegenerators 140, 141 and 142, CPU in the control board 130 executes aninterrupt operation to detect the rotational speed Ne of the engine, therotational speed Nt of the output shaft of the subtransmission and therotational speed No of the wheel driving shaft, reads the open or closedcondition of the various input switches and detecting switches(representing the presence or absence of various commands), reads thewater temperature signal, the oil temperature signal and the throttlevalve opening signal after their analog-to-digital conversion,establishes a speed range as indicated in FIG. 3, performs a shiftoperation to the particular speed range, detects any abnormalityoccurring in the transmission, detects any abnormality in the water oroil temperature, and illuminates the abnormality indicator lamp 151 in aflashing pattern which corresponds to the particular abnormalitywhenever such abnormality has been detected.

When establishing a speed range or shifting to a different speed range(see FIG. 3) and when activating and deactivating the lockup, CPU in thecontrol valve 130 performs such operation by changing and regulating thepressures supplied to various clutches and brakes through an outputinterface thereof, namely turning the solenoid valves SL1 to SL3 eitheron or off, turning the solenoid valves SL4 and SL5 either on or off orcontrolling their duty cycle, and controlling the duty cycle of thecurrent flow through the linear solenoid valve SL6 in accordance withthe opening θ of the throttle valve and the vehicle speed No.

The control board 130 includes a memory backup power circuit whichmaintains a memory activating voltage applied to CPU even if theignition key switch IGS is open, thus retaining data which is writteninto the internal memory of CPU. The input switches, lamps and varioussensors are connected to a signal processing circuit which is fed from asingle power circuit which is in turn fed from batteries 163, 164through the ignition key switch IGS. On the other hand, solenoid valves,SL1 to SL6, are connected to the output terminals of solenoid driverswhich are fed from a high power circuit which is in turn fed from thesame batteries 163, 164 through the ignition key switch IGS. A givenvoltage is applied to a control signal input circuit of each of thesolenoid drivers from the signal power circuit. Accordingly, during thetime the ignition key switch IGS remains open, the power consumptionfrom the battery by the memory backup power circuit and the CPU isminimal.

FIG. 5a to 5e are flow charts of a main routine of CPU contained in thecontrol board 130 in exercising its control operation. When a powersupply to the control board 130 is turned on at step 1 shown in FIG. 5a(hereafter a particular step or subroutine will be designated by anassociated numerical figure alone without using the term "step" or"subroutine"), thus connecting the battery 163 to the control board 130.Internal registers, timers and counters within CPU will be reset totheir initial standby conditions while standby signal levels aredelivered to output ports (2). In this manner, SL1 to SL6 are all turnedoff or deenergized.

The battery 164 is connected in the manner illustrated in FIG. 4, andsubsequently when the ignition key switch IGS is closed and as long asit remains closed, CPU performs and repeats a control operation of onecycle, represented by steps 3 to 69 shown in FIGS. 5a to 5e, with a TPperiod.

At the outset of the control operation of one cycle, a timer TP isstarted (3). Subsequently, input switches, throttle valve opening sensorand the like are read (4). During a data processing 1 (5), inputs whichare read in this manner are written into registers to which referencewill be made during the execution of a control program, and variousparts are examined if there is any indication of abnormality on thebasis of such input data. Whenever an abnormality is detected, the lamp151 is illuminated.

During data processing 2 (6), CPU in the control board 130 thendetermines, on the basis of the shift lever position, a current speedrange which is currently established by the automatic transmission shownin FIG. 1 and depending if the fuel saved running command switch 132 isturned on or off, a possible shift mode or a next speed range to which ashift up/down from the current speed (which is indicated by the contentof the current speed range register PS) can be made. In the presentembodiment, a reference vehicle speed (actually, a group of referencevehicle speed values) having a throttle valve opening as a parameter andwhich is used in determining the need to make a shift from a particularspeed range (for example, a third speed) to another speed range (afirst, a second or a fourth speed range) is provided in two sets,namely, one used when the fuel saved running is specified (economy modedata group: FIG. 15b) and another used when such running is notspecified (power mode data group: FIG. 15a). Accordingly, during thedata processing 2 (6), data indicating a possible shift mode from thecurrent speed range is determined as well as data indicating if theeconomy mode is or is not employed are used to specify a particularreference vehicle speed group.

On the basis of the input data which is read at the input reading (4)and written into the registers at the data processing 1 (5), CPU in thecontrol board 130 then examines (7, 11) if a 1.5 speed range (LS) is tobe established in consideration of the correlation between the shiftlever position (output from the switch 134) and the presence or absenceof a command for the 1.5 speed range (LS) (open or closed condition ofthe switch 131). In the present embodiment, the 1.5 speed range isestablished only in the L position of the shift lever and when theswitch 131 is closed. In this instance, 1 (representing the need toestablish the 1.5 speed range) is written into register LSF (12).Otherwise, the register LSF is cleared (13).

During a shift decision (14), a reference is made to the current speedrange (the content of current speed range register PS), the content ofregister LSF and the input data from the fourth speed inhibit switch133, data representing the 1.5 speed range is written into a targetspeed range register DS if the content of the register LSF is equal to1, indicating the need to establish the 1.5 speed range. If the contentof register LSF is equal to 0, indicating no command, a reference ismade to data which specifies a particular reference vehicle speed group.Such data comprises data indicating a possible shift mode from thecurrent speed range and data indicating if the economy mode is employedor not. Initially, a particular reference vehicle speed group (one ofsolid line curves shown in FIGS. 15a and 15b) is specified from ahighest one (SRi speed range) of speed ranges, to which an up shift fromthe current speed range (the content of register PS) can be made, andfrom which the first speed range is excluded when the inhibit switch 133is closed. From this group, the value of a particular speedcorresponding to the current throttle valve opening, representing asingle point on the specified solid line curve, is selected, and iscompared against a current vehicle speed No. If the vehicle speed No isequal to or greater than the reference vehicle speed value, indicatingthe need for an up shift, data indicating SRi speed range is writteninto the target speed range register DS. Otherwise, such entry is notmade, but a similar decision is repeated for a next lower speed range.If the need for an up shift is not found to any of speed ranges higherthan the current speed range, a particular reference vehicle speed group(one of broken line curves shown in FIGS. 15a and 15b) is specified froma lowest one (SRj) of speed ranges, to which a down shift can be madefrom the current speed range. From this group, a value of a particularreference speed representing a single point on the specified broken linecurve which corresponds to the current throttle valve opening isselected, and is compared against the current vehicle speed No, and ifthe vehicle speed No is equal to or less than the reference vehiclespeed, indicating the need for a down shift, data indicating SRj speedrange is written into the target speed range register DS. If thereference vehicle speed is exceeded, such entry is not made, but asimilar decision is repeated for a next higher speed range. It is to benoted that when the switch 133 is closed, representing an inhibit of thefourth speed range, and if the content of the current speed rangeregister PS indicates the fourth speed range, the third speed range iswritten into the target speed range register DS in order to perform adown shift from the fourth to the third speed range.

CPU in the control board 130 then compares the current speed range (thecontent of register PS) and the content of the target speed rangeregister DS (a speed range to be established) against each other, and inthe event they do not compare, indicating the need for a shiftoperation, writes the particular speed range stored in the target speedrange register DS into a next speed range register SS and a nextfollowing speed range register SSN (15-16-17-18) unless a shiftoperation is not currently being performed. However, if it is in theprocess of a shift operation now, the speed range in the register DS iswritten into only the next following speed range register SSN(15-16-17-36).

If it is not in the process of a shift operation now, a timer TB isstarted (19), and 1, indicating the timer TB in operation, is writteninto a register TBF (20). When the timer TB times out, the solenoidvalves SL1 to SL3 and the timing solenoid valve SL5 are energized inorder to establish a particular speed range which is written into thenext speed range register SS. A timing controlling timer is started inorder to achieve a smooth transition of oil pressures to the clutchesand brakes during a shift operation, and data is entered into variousregisters (44 to 54).

The detection of shift time (TSO, TSE) (64), the control of the linearsolenoid valve SL6 (65), the control of the timing solenoid valve SL5(66), the lockup control (67) and the output control (68) aresequentially executed in the sequence named. When the timer TP times out(69), the timer TP is started again (3), initiating the controloperation of the next one cycle. By repeating the control operation ofone cycle with the TP period, the decision for a shift operation and thecontrol over the up/shift down when the need for a shift operation isfound can be smoothly achieved in a time sequence.

The detection (64) of the shift time (TSO, TSE) shown in FIG. 5d isillustrated in detail in FIGS. 6a and 6b, the control (65) of the linearsolenoid valve SL6 shown in FIG. 5d is illustrated in detail in FIGS. 7aand 7b, and the control (67) of the timing solenoid valve SL5 shown inFIG. 5d is illustrated in detail in FIGS. 8a, 8b and 8c. Nomenclaturesappearing in these Figures are described below.

LSF: A register for storing information representing the presence orabsence of a command for the 1.5 speed range. When its content is "1",there is a command for the 1.5 speed range. Accordingly, "0" indicatesthe absence of such command.

PS: A current speed range register having a content which indicates acurrent speed range of the automatic transmission shown in FIG. 1,including a parking P, a back R, a neutral N condition, a first, a 1.5,a second, a third or a fourth speed range.

DS: A register for temporarily storing a speed range (a first, 1.5, asecond, a third or a fourth speed range) which is determined by theshift decision to be established.

SS: A next speed range register for storing a speed range (a first, 1.5,a second, a third or a fourth speed range) which is to be established. Ashift mode is determined by the content of the registers PS and SS.

SSN: A next following speed range register for storing a speed rangewhich must be established following a shift operation from PS to SS.

TBF: A flag register indicating if the timer TB is or is not inoperation. Its content "1" indicates that the timer is in operation, and"0" indicates no operation, or that the tiemr TB has not been started.

TEF: A flag register indicating if the timer TE is or is not inoperation. Its content "1" indicates that the timer is in operationwhile "0" indicates no operation, or the timer TE has not been started.

PUF: A flag register indicating a power on up shift. Its content "1"indicates that this is an up shift or shift to a higher speed rangeunder the condition that a vehicle drive load is applied to the enginewhile "0" indicates a down shift or a shift to a lower speed range or anup shift under no load condition upon the engine (power off, meaningthat the vehicle is coasting or an engine brake is being applied).

TEIF: A flag register indicating the beginning of a shift time. Itscontent "1" indicates that it is now immediately after the timer hasbeen started.

TEFF: A flag register indicating that it is now immediately after thetermination of the shift period. Its content "1" indicates that it isnow in a time interval which follows the time-out of the tmer TE, but isbefore a timer T2D times out.

i, j: A number of times register in which the number of times is enteredthat a power on condition (a vehicle driving load is applied to theengine) or not (power off) has been detected.

k: A number of times register for storing a number of times theinequality Nt1>Nt2 applies (where Nt2 represents a current Nt while Nt1represents the value of Nt which prevailed one TP previously), which isused in determining the beginning of a shift operation.

TTF: A flag register indicating the completion of the determination ofthe shift time. Its content "1" indicates that the determination of theshift time TT has been completed while "0" indicates that thedetermination of the shift time has not yet been completed.

TCR: A flag register indicating an abnormality of the shift time TT whenit has a value of "1".

CR: A register into which data is entered indicating the degree ofsuitability of the shift time TT.

POI: A flag register indicating the need to establish an initial valuefor the duty cycle of the linear solenoid valve SL6, indicating thetermination of establishing the initial value when it has a content of"1".

ADMEM: A register for storing a learned value of the duty cycle for thelinear solenoid valve SL6.

ADIN: A register for storing a calculated value of the duty cycle forthe linear solenoid valve SL6.

ATIN: A register for storing a calculated value of the duty cycle forthe timing solenoid valve SL5.

The control operation of this embodiment will now be described indetail.

(1) Detection of Ne, Nt, No

Immediately after the ignition key switch IGS is closed, CPU in thecontrol board 130 starts a program timer having three time limits(timers 1 to 3), enabling an interrupt operation responsive to a pulsegenerated by either pulse generator 140, 141, 142. For example, inresponse to the falling edge of a pulse generated by the generator 140,CPU enters an interrupt operation in which a count register 1 isincremented by one, followed by examining if the timer 1 has or has nottimed out. If the timer has not timed out, CPU returns to a particularcontrol operation which it assumed before entering the interruptoperation. If the timer 1 has timed out, the content of the countregister is written into a register Nef used for calculating a speed Newhile the timer 1 is re-started and CPU returns to the previous control.In response to the falling edge of a pulse generated by the generator141, CPU enters an interrupt operation in which a count register 2 isincremented by one, followed by examining if the timer 2 has or has nottimed out. If the timer has not timed out, CPU returns to its controlwhich it assumed before the interrupt operation is entered. If the timer1 has timed out, the content of the count register 2 is written into aregister Ntf for calculating a speed Nt. The timer 2 is re-started andCPU returns to its previous control. In response to the falling edge ofa pulse generated by the generator 142, CPU enters an interruptoperation in which a count register 3 is incremented by one, followed byexamining if the timer 3 has or has not timed out. If the timer has nottimed out, CPU returns to its previous control which it assumed beforeentering the interrupt operation. If the timer 3 has timed out, thecontent of the count register 3 is written into a register Nof forcalculating a speed No, the timer 3 is re-started and CPU returns to theprevious control.

As a result of the execution of these interrupt operations, the numberof pulses generated by the respective pulse generators 140, 141 and 142during a latest given time limit is written into the registers Nef, Ntfand Nof. During data processing 2 (6) shown in FIG. 5a, CPU in thecontrol board 130 calculates speeds Ne, Nt and No on the basis of datastored in the registers Nef, Ntf and Nof, representing the number ofpulses generated in a given time limit, and write them into speedregisters Ne, Nt and No, respectively. In this manner, the latest orupdated speed data is maintained in these speed registers.

(2) A summary of control timing from determining the need for a shiftoperation until the shift operation is completed.

For an up shift, the timer TB is started (19) when the need for a shiftoperation is determined, as indicated by a white triangle in FIG. 9a.When the timer times out, the energization of the solenoids SL1 to SL3are switched so as to establish a next speed range (43 to 48). Thistiming is indicated by a black triangle in FIG. 9a, indicating "shiftoutput". For example, for a 2→3 shift, the energization/deenergizationof the solenoids SL1 to SL3 is controlled according to D-3 row shown inFIG. 3. The timer TE is then started (46).

In this embodiment, TB=0.2 sec. TE is chosen to be equal to 0.8 sec fora 4→3 down shift and equal to 1.5 sec for other up shifts and downshifts. Such TE value is chosen to be greater than a sum TSE of a timeTSO which passes from the delivery of the shift output (48) to theinitiation of the actual mechanical shift operation and a time TT fromthe initiation of the mechanical shifting to the completion thereof. Itis to be noted that a mechanical shift time represents the actual shifttime.

(3) Determination of mechanical shift time TT.

A mechanical shift time TT varies with the abrasion of frictionalmembers of the automatic transmission shown in FIG. 1 and with a loadingassociated with a running of the vehicle. If such time is too short, ashift shock is likely to occur. On the other hand, if such time is toolong, a blow-up of the engine or a poor acceleration may result. In viewof such drawbacks, the mechanical shift time is a measure of the meritof the automatic transmission.

In the present embodiment, for a 1→2 or 2→3 up shift, the determinationof the mechanical shift time TT is made after starting the timer TE. Thedetermination is illustrated in detail in FIG. 6a. Specifically, theinitiation of a mechanical shift is determined to have occurred upondetecting a reduction in the speed Nt consecutively for two or more TPperiods, and the determination of the mechanical shift time TT isinitiated (81 to 87). A mechanical shift is determined to have beenterminated in unit of TP period in response to a reduction of a new Ntvalue which is 2.5 rpm or more less than a previous Nt value. The valueTT thus determined is written into the register TT (88 to 90) and 1 iswritten into the flag register TTF (91) in order to indicate thetermination of the determination of the time TT.

(4) Detection of a failure in the transmission by means of themechanical shift time TT.

When a loading associated with a running of the vehicle is abnormallyhigh, when frictional members such as clutches or brakes in theautomatic transmission shown in FIG. 1 are exposed to a greater degreeof abrasion, when some kind of abnormality develops within atransmission mechanism or when the vehicle runs in an abnormal manner,or a failure or abnormality within the transmission mechanism is found,the mechanical shift time TT will become excessively long or excessivelyshort, producing a high probability of causing a shift shock or ablow-up of the engine. To accommodate for this, in the present example,eight zones to 1 to 8 are determined about a basic shift time (fixedvalue) TS utilizing the throttle valve opening θ as a parameter, asshown in FIG. 10d. A shift time TT which is determined is examined tosee in which zone it lies (92 in FIG. 6b), and if it lies in a zone 1 or8, 1 is written into the abnormality register TCR (103), indicating anabnormality (104) by energizing the lamp 151. If the shift time TTdetermined lies in a zone 2 or 7, the number of times An such fact isfound is counted. When the number of times An is less than 4, 2 iswritten into the degree of suitability register CR (99 to 102). When thenumber of times is equal to or greater than 4, an operation occurs inthe similar manner as when the shift time lies in a zone 1 or 8. Whenthe shift time lies in a zone 3 or 6, the abnormality is cleared (96,97), and 1 is written into the degree of suitability register CR (98).Also when the shift time lies in a zone 4 or 5, the abnormality registeris cleared (93, 94), and 0 is written into the degree of suitabilityregister CR (95), by clearing this register. Accordingly, data inregisters CR and TCR represent the degree of suitability of a shifttime.

(5) Multiple shift.

If there is a change in the throttle valve opening or the vehicle speedNo during a time interval (TB+TE) from the decision of a need for ashift operation until the shift operation is actually completed, it maybecome necessary to shift to a speed range which is different from thespeed range updated during the interval (TB+TE).

When the need for a shift operation is determined during an interval TB,meaning the time interval from starting the timer TB until it times out,a speed range which is found necessary to be established is written intothe next speed range register SS (23 in FIG. 5b). As a result, the speedrange which had been written into the register SS immediately before theTB interval occurs is now erased as a result of this writing, and hencethe shift output available for the speed range when the timer TB timesout (48 in FIG. 5c) is effective to establish a speed range which isdetermined during the TB interval.

If the need for a shift operation is determined during the shift time orduring the time the timer TE is in operation, as indicated by TEF=1,subsequent to the TB interval (15-16-17), it is to be noted that theimmediately preceding shift operation has been completed. Accordingly,the speed range which is to be established next is written into the nextfollowing speed range register SSN (36), and as the timer TE times out,a shift output for causing the speed range for which the need of a shiftoperation is found is delivered (41-42-55-56-57-58-44 to 48). In otherwords, the shift to the speed range takes place immediately followingthe previous TE, without interposing another TB interval.

(6) Determination of power on up shift.

A shift shock is likely to occur during a shift (TE) in the power on upshift, and a blow-up of the engine is likely to occur during a power offup shift. Accordingly, during an up shift (TE), the rate at which theoil pressure is changed is determined in a manner corresponding to poweron/off, thus preventing the occurrence of a shift shock or an engineblow-up. At this end, the detection of a power on/off is necessary. Thispreferably takes place immediately before the shift (TE) since thethrottle valve opening θ, vehicle speed No, the number of revolutions ofthe engine Ne and the number of revolutions Nt of the output shaft ofthe subtransmission 2 change from time to time. Accordingly, in thepresent embodiment, the detection of power on/off is made during the TBinterval (16-23 to 35). It is to be noted that the decision of the poweroff also takes place during the TE interval (17-36-30 to 35).

Specifically, if Ne≧Nt applies during the TB interval (TBF=1) when theshift mode is an up shift, the content of the number of times register iis incremented by one (16-23-24-27-28-29). If the content of theregister i is equal to or greater than 2, 1 representing the power on upshift, is written into the flag register PUF (31). If this is in the TBor TE interval, the content of the number of times register j isdecremented by one when Ne<Nt applies (32, 33), and when the content ofthe register j is equal to or greater than 2, the flag register PUF iscleared (35). When it is cleared, this means that the power off modeprevails. In this manner, when the shift mode is an up shift, the poweron or off is determined consecutively until the timer TB times out,whereupon the power off is determined. When the power on is determined,the content of the register PUF is established to be equal to 1 while itis equal to 0 if the power off is determined. Accordingly, whendelivering the shift output (48), information indicating if the power onprevails or not is stored in the register PUF.

(7) Steady-state control of the duty cycle for the linear solenoid SL6(FIGS. 7a and 7b).

The linear solenoid SL6 develops an oil pressure which is substantiallyproportional to the magnitude of the current flow through its electricalcoil, and an oil pressure which is proportional to this oil pressure isapplied by the accumulator pressure control valve 110 as the piston backpressure to the accumulators 260 to 290. The relationship between themagnitude of the current flow through the linear solenoid SL6 and theback pressure on the piston of the accumulators 260 to 290 is showngraphically in FIG. 9b. In this embodiment, the magnitude of a currentflow through the linear solenoid SL6 is determined by the duty cycle.The relationship between the duty cycle and the average current flow isindicated also in FIG. 9b.

The linear solenoid SL6 is used as a substitute for a conventionalthrottle valve used in the conventional hydraulic circuit which ismechanically coupled to the rotary shaft of a throttle valve and isresponsive to a governor pressure which is in turn dependent upon therotational speed of the engine to regulate a line pressure to a valuecorresponding to the throttle valve opening and the governor pressure.At a given timing, principally during TE interval, in a particular shiftmode, CPU in the control board 130 controls the duty cycle forcontrolling the back pressure as mentioned in the next followingparagraph (8) in order to control the back pressure of the accumulators260 to 290 to thereby prevent the occurrence of a shift shock. However,at other times, CPU causes an energization at a current level (or with acorresponding duty cycle) as shown in FIG. 9c in a manner correspondingto the throttle valve opening θ and the rotational speed Nt of theoutput shaft of the subtransmission 2. Specifically, a pressure whichcorresponds to the throttle valve opening θ and the rotational speed Ntis applied to the accumulators 260 to 290 and the 2→3 shift valve 60(115, 116 in FIG. 7a).

(8) The back pressure control of accumulators during a shift operation(FIG. 7).

For a 1→2, 2→3 or 3→4 power on up shift (PUF=1), in order to prevent theoccurrence of a shift shock, during the shift interval (TE: see FIG.9a), if it now prevails, the duty cycle of the linear solenoid SL6 orthe back pressure on the piston of the accumulators 260 to 290 issubstantially determined as follows:

    K1×K2×[K3 (1-TT/TS)+ADMEM]

where K1 represents an environmental change correction factor and isgiven by K1=K11+K12 where K11 represents an oil temperature correctionfactor and K12 a correction factor corresponding to the throttle valveopening, K2 is a correction factor corresponding to the shift mode andthe throttle valve opening, and K3 is a correction factor correspondingto the shift mode. The values of these correction factors are shown inFIGS. 10a, 10b, 10c and 10d. The correction factor K11 is calculated ina manner corresponding to the temperature detected by the oiltemperature sensor 136 (118 in FIG. 7b). The correction factor K12 iscalculated in a manner corresponding to the throttle valve opening θ(118). The correction factor K2 is calculated in a manner correspondingto the throttle valve opening θ and the shift mode (120). The correctionfactor K3 corresponds by a one-to-one relationship to the shift mode,and hence a value corresponding to the shift mode or the content of theregisters PS and SS is selected.

TT represents the latest mechanical shift time TT, or data which isdetected by the flow chart shown in FIG. 6a and stored in the registerTT while TS represents a basic shift time (fixed value). ADMEMrepresents the duty cycle which is determined by the correction whichtakes place as a result of learning effect.

In calculating

    K1×K2×[K3 (1-TT/TS)+ADMEM]

mentioned above, CPU in the control board 130 calculates this asfollows. Initially, the learned value ADMEM or the content of theregister ADMEM, is modified into a proper value [K3 (1-TT/TS)+ADMEM] ina manner corresponding to the latest mechanical shift time TT and theprevailing shift mode, and is then written into register ADMEM to updateit (126). The current environmental factor K1 (=K11+K12) and thecorrection factor K2 are multiplied by the data stored in the registerADMEM, and the product is delivered as an output duty cycle to bewritten into an output data register ADIM which is directed to thelinear solenoid SL6 (128). During the output control (68 in FIG. 5d),CPU in the control board 130 delivers an on/off signal having the dutycycle represented by data stored in the output data register ADIM to thesolenoid driver which is associated with the energization of thesolenoid SL6.

When the batteries 163 and 164 are connected to the control board 130,the control board 130 is fed with a power supply, and subsequently whenthe ignition key switch IGS is closed, a control over the automatictransmission shown in FIG. 1 is enabled for the first time. When soenabled, there is no learned value ADMEM for the duty cycle of thelinear solenoid SL6. Accordingly, the content of register POI is now 0.Accordingly, when the content of the register POI is equal to 0, aninitial value (fixed value) is written into the register ADMEM to write1 into the register POI, indicating that the initial value has beenestablished (122, 123). Since the learned value must not be updated(126) at this time, no updating takes place, and ADMEM multiplied by theenvironmental factor is used to determine the duty cycle (124).

As mentioned above under the paragraph (4), in the event a failure ispresent within the transmission, the shift time TT will be displacedfrom the reference value TS. Accordingly, the modification of thelearned value for the duty cycle (126) is inappropriate. Hence areference is made to the degree of suitability of the shift time TT (thecontent of the registers TCR and CR), and if the shift time TT lies ineither one of the zones 1, 8, 2 or 7 (FIG. 10d), a modification of thelearned value (126) is not made, but the prevailing duty cycle (ADMEM)is corrected by an amount corresponding to a change in the environmentto determine the duty cycle for the linear solenoid SL6 (125-128). Thus,only when the shift time TT lies in one of zones 3 to 6, a modificationof the learned value of the duty cycle is made. This assures a smoothautomatic pressure regulation (or correction of the duty cycle) as atime sequence only within a proper range of the shift response (TT) ofthe automatic transmission so as to bring the shift time TT closer tothe proper value TS.

FIG. 11a graphically shows a change, in a time sequence, in the oilpressures of the brake B₁ and the clutch C₂ during a 2→3 shift. In thisFigure, the oil pressure of the brake B₁ is shown in solid line forpower on and in broken lines for power off. The oil pressure of theclutch C₂ for power on is shown in a thick solid line, in two dots andsingle dot lines while the oil pressure during the power off is notshown.

Depending on the duty cycle of the linear solenoid which is achieved bythe back pressure control of the accumulator which is executed onlyduring the power on, the oil pressure of the clutch C₂ rises rapidly andthe shift time TT will be short as indicated by two-dots lines when theduty cycle is low. On the contrary, for a high duty cycle, the rising ofthe oil pressure of the clutch C₂ takes place slowly, as indicated bysingle dot line, causing an increase in the shift time TT. During thepower off, the duty cycle is rendered to be equal to 0 during the shiftoperation, and accordingly the oil pressure of the clutch C₂ rises mostrapidly, and the neutral interval shown will be reduced than that shownin the drawing.

It will thus be seen that the mechanical shift time TT depends on theduty cycle, assuming a longer value for a higher duty cycle and assuminga shorter value for a lower duty cycle. During the back pressure controlof the accumulator, a correction+K3 (TS-TT)/TS, or a correctioncorresponding to a deviation of the actual shift time TT relative to thereference value TS is added to the previous duty cycle to provide thenext duty cycle (126 in FIG. 7b), thus performing a modification of thelearned value. In this manner, the mechanical shift time TT converges tothe reference value TS. In this manner, the duty cycle is automaticallyadjusted in accordance with a change in the engaging responses of thefrictional members such as the brake (B₁) and the clutch (C₂) due totheir abrasion to maintain the shift time virtually at the referencevalue TS, thus preventing the occurrence of a shift shock.

(9) Control of timing solenoid SL5 during shift operation (FIGS. 8a, 8band 8c).

The timing solenoid SL5 determines the rate at which the oil pressureapplied to the brake B₁ for its engagement rises and the rate at whichthe oil pressure of the brake B₁ falls as it is released. The brake B₁is engaged only in the second speed range (see FIG. 3), and accordingly,the rising or falling rate of the oil pressure of the brake B₁ iscontrolled in a shift mode which relates to the oil pressure of thebrake B₁, thus preventing the occurrence of a shift shock.

For a 2→3 shift shown in FIG. 11a, the timing solenoid SL5 continues tobe turned off for the power on (131-132-135-145-return in FIG. 8a),whereby the bypass valve 80 causes the oil path which bypasses theorifice 82 to be closed. Accordingly, since the brake B₁ communicateswith the 2→3 shift valve 60 which connects it to the drain (lowpressure) through the orifices 81 and 82, the rate at which the brake B₁is decompressed is low. If a change from the power on (PUF=1) to thepower off (PUF=0) occurs during a time interval after the TE interval isentered and until 0.1 sec passes thereafter, CPU in the control board130 turns on the timing solenoid SL5 when the timer T1 times out(132-145 to 147: a rising shown in broken lines and occurring 0.1 seclater shown in the SL5 line in FIG. 11a). Specifically, data specifyinga duty cycle of 100% is written into the output register ATIN associatedwith the timing solenoid SL5 (147). The time limit T1 (0.1 sec) may bechanged to different values in a range from 0.1 to 0.3 sec. When thetiming solenoid SL5 is turned on, the bypass valve 80 opens the oil pathwhich bypasses the orifice 82, so that the brake B₁ will be decompressedat a higher rate. When the TE interval passes, the timing solenoid SL5is turned off or its duty cycle is changed to 0% (131-149-156 in FIG.5b-157-159). In FIG. 11a, a change in the pressure of the brake B₁during the power on is shown in solid line. At the output (68 in FIG.5d), CPU in the control board 130 refers to data stored in the outputregister ATIN associated with the timing solenoid SL5, and delivers anon/off signal having the duty cycle indicated by such data to thesolenoid driver associated with the solenoid SL5.

During a 2→3 shift, if this is the power off, CPU in the control board130 turns on the timing solenoid SL5 (to the duty cycle of 100%) uponentering the TE interval (134) and turns it off (to the duty cycle of0%) after the TE interval has passed (131-149-156 in FIG. 5b-157-159).

The timing for energizing the timing solenoid SL5 during a shift to thesecond speed range with the power on is illustrated in FIG. 11c. Duringa shift to the second speed range, the throttle valve opening θ and therotational speed No of the output shaft of the main transmission 3 areexamined to determine in which one of regions I to IV and non-controlledregion, both shown in FIG. 11b, their combination lies during aninterval A which covers a time interval of 0.4 sec upon entering the TEinterval (131-132-135 to 139). If the combination is found to lie inregion I, the duty cycle for the timing solenoid SL5 is chosen to be100% (140). The duty cycle will be chosen to be 65% when the combinationlies in the region II (141), to 50% for region III, and 0% (off) forregion IV (142). FIG. 11d shows an actual demarcation of the regions Ito IV, and FIG. 11e graphically shows the relationship between theregion in which the combination is determined to lie and thecorresponding duty cycle. In an interval B (FIG. 11c) which lies in theTE interval but the initial 0.4 sec has passed, a determination is madein which one of the regions I to IV and non-controlled region shown inFIG. 11b (or more exactly, FIG. 11d) the combination of the throttlevalve opening θ and the rotational speed No of the output shaft of themain transmission 3 lies. A difference, expressed in terms of suchregion, between a region As which is determined to contain thecombination and a region Ap which was previously determined, is examined(138). If the difference corresponds to two regions or greater, the dutycycle is determined so as to correspond to the new region As (139-140 to143). If the difference expressed in terms of the regions is equal to 1or less, the duty cycle remains unchanged (133-144). A change in thepressure of the brake B₁ which results from such duty cycle control isillustrated graphically in FIG. 11f.

For the power off up shift, the timing solenoid SL5 is turned on (to theduty cycle of 100%) for a 2→3 or 2→4 shift (132-145-146-133-134).

For a 2→1 shift, the timing solenoid SL5 is maintained off (or to theduty cycle of 0%) until the completion of the shift operation or untilthe TE interval passes, and the timing solenoid SL5 is turned on (to theduty cycle of 100%) upon completion of the shift operation, asgraphically illustrated in FIG. 11g. Accordingly, the oil pressure ofthe brake B₂ will assume a high value for transmitting the maximumtorque in the first speed range after TE (1.5 second). Since the oilpressure is changed to one used in the first speed range at TE after theshift operation, the oil pressure of the brake B₂ is low immediatelyafter the shift operation or within the TE interval, preventing theoccurrence of a shift shock.

(10) Control of energization of the timing solenoid SL5 upon changingthe shift lever from D or S position to L position (FIG. 8a).

When the shift lever changes from D or S position to L position, CPU inthe control board 130 writes 1 into a manual shift detecting register,and starts a timer T20a having a time limit T20a (2 sec)(131-149-150-151). When the timer times out, CPU turns the timingsolenoid SL5 on (to the duty cycle of 100%), and clears the manual shiftdetecting register (131-149-150-152-153 to 155). As a result of this,upon changing the shift lever from D or S position to L position, theoil pressure of the brake B₂ will rise to a value for transmitting themaximum torque used in the first speed range after T20a (2 seconds)thereafter. Since the oil pressure of the brake B₂ has been maintainedlow until that time, no shock results from a change in the oil pressure.The occurrence of a shift shock is also prevented if a shift to thefirst speed range occurs in connection with changing the shift lever toL position.

(11) Shift control relating to 1.5 speed range (LS).

The first to the third speed range is established by maintaining thesubtransmission 2 low (SL3: off; the term "high/low" refers to a rangeof rotational speed of the output shaft of the subtransmission 2, and"low" corresponds to a high gear ratio) and by a speed range establishedin the main transmission 3 which is determined by the combination ofon/off of the solenoids SL1 and SL2. The fourth speed range (O/D) isdetermined by maintaining the subtransmission 2 high (SL3: on,corresponding to a low gear ratio) and establishing the third speedrange (SL1, SL2: off) in the main transmission 3. Accordingly, a shiftbetween the first to the third speed range actually means a switchingbetween speed ranges of the main transmission 3 (single change shift).

However, 1.5 speed range is determined by maintaining thesubtransmission 2 high (SL3: on) and by establishing the first speedrange in the main transmission 3 (SL1: off/SL2: on). Accordingly, ashift between 1.5 speed range on one hand and the second and the thirdspeed range on the other hand is a double change shift requiring theswitching of both the main transmission 3 and the subtransmission 2.

1→1.5 shift

FIG. 12a shows the timing of controlling the solenoids during a 1→1.5shift. In this shift mode, the main transmission 3 remains in the firstspeed range while the subtransmission 2 is switched from its lowcondition (SL3: off) to its high condition (SL3: on), thus representinga single change shift. During this shift mode, the timing solenoid SL5continues to be turned on (to the duty cycle of 100%). The solenoids SL1to SL3 are changed from their on/off setting which is used to establishthe first speed range to that which establishes 1.5 speed range (seeFIG. 3) upon entering the TE interval or when the timer TB has timedout. Specifically, SL3 is changed from its off to its on condition.

The linear solenoid SL6 is energized with a duty cycle or with a currentvalue shown in FIG. 9c which corresponds to the throttle valve opening θand the rotational speed Nt of the output shaft of the subtransmission 2until the end of the TB interval, but upon entering the TE interval, theduty cycle is fixed to a value which prevailed immediately before. Inother words, the energization takes place with a fixed duty cycle duringthe TE interval.

1.5→2 shift

FIG. 12b shows the timing of controlling the solenoids during a 1.5→2shift. This shift mode represents a double change shift in which themain transmission 3 is changed from the first to the second speed rangeand the subtransmission 2 is changed from its high to the low condition.During this shift mode, the timing solenoid SL5 is turned off (to theduty cycle of 0%) at a time when this shift is determined (15 to 22 inFIG. 5b), and when the TB interval has passed, a determination of theparticular region in which the combination lies is made as mentionedpreviously with reference to FIG. 11b, and the solenoid is energizedwith a duty cycle corresponding to the region thus determined (148-136to 144 in FIG. 8b). The linear solenoid SL6 is controlled in the similarmanner as in the 1→1.5 shift mentioned above.

The shift solenoids are changed in this shift mode as follows: SL1: off,SL2: on, SL3: on (these conditions prevail in the 1.5 speed range) toSL1: on, SL2: on, SL3: off (these conditions prevail in the second speedrange). Upon entering the TE interval, CPU in the control board 130initially turns on both solenoids SL1 and SL2 in preparation toestablishing the second speed range, but retards the switching of thesolenoid SL3 to its off condition. Subsequently, CPU calculates adifference or mismatch between the main transmission and thesubtransmission from their synchronization AN=Nt-1.53 No between therotational speed Nt of the output shaft of the subtransmission 2 and arotational speed 1.53 No which will be produced at the input shaft ofthe main transmission 3 (or the output shaft of the subtransmission 2)when it is at No. On the other hand, CPU selects a reference value (Gmapvalue: FIG. 13) having the rotational speed No as a parameter and turnsthe solenoid SL3 off at the time (point X) when the inequality ΔN≦Gmapapplies in order to establish the second speed range. FIG. 13a showsvalues of Gmap.

In the 1.5 speed range, the subtransmission 2 is maintained high in thesimilar manner as in O/D (the fourth speed range) and the maintransmission 3 is determined in the same manner as in the first speedrange. Accordingly, a 1→1.5 shift and a 1.5→1 shift can be achieved bymerely changing the subtransmission from its low to its high conditionor conversely. However, a 1.5→2 shift or a 1.5→3 shift requires that thesubtransmission 2 be changed from its high to its low condition and thatthe main transmission be changed from the first to the second or thethird speed range, thus giving rise to the likelihood that shift shocksmay be developed in two steps (for each of the two transmissions).Accordingly, during the 1.5→2 shift, the solenoid SL3 is turned off atthe time (point X) when ΔN≦Gmap applies in order to establish the secondspeed range. The purpose of this is to achieve a substantialsynchronization of the completion of shift operations in both thesubtransmission 2 and the main transmission 3.

FIG. 13b graphically shows the timing chart for the 1.5→2 shift in moredetail. In the 1.5→2 shift, when the solenoid SL3 is turned off at theΔN value shown in FIG. 13c (it will be noted that since SL1 is on as isSL2, turning SL3 off establishes the second speed range), shiftoperations in the main transmission 3 and the subtransmission 2 will becompleted in substantially synchronized manner, thus preventing theoccurrence of shift shocks. ΔN value which achieves such synchronizingcharacteristic has No as a parameter as indicated in FIG. 13c, and Gmaphas a value which is close to ΔN value as indicated in the rightmostcolumn of FIG. 13c (and FIG. 13a). Since the point X at which thesolenoid SL3 is turned off is chosen so that the actual value of ΔN isnot greater than Gmap value (reference value), it follows that shiftoperations in the main transmission 3 and the subtransmission 2 arecompleted in substantially synchronized manner, substantially preventingthe occurrence of shift shocks.

1.5→3 shift

This shift mode is a double change shift, requiring changing the maintransmission 3 from the first to the third speed range and changing thesubtransmission 2 from its high to its low condition. Such control issimilar to the control which is used during the 1.5→2 shift mentionedabove, but in this shift mode, it is chosen that ΔN=Nt-1.00×No.

1.5→4 shift

The control timing during this shift mode is schematically shown in FIG.12c. This shift mode is a single change shift in which the maintransmission 3 is changed from the first to the third speed range whilethe subtransmission 2 is maintained high. When the need to choose thisshift mode is found, the timing solenoid SL5 is turned off (to the dutycycle of 0%) upon starting the timer TB (15 to 22 in FIG. 5b), and ismaintained off after the completion of the shift operation or upontermination of the TE interval. The on/off condition of the solenoidsSL1 to SL3 are changed upon entering the TE interval. The control of theduty cycle of the linear solenoid SL6 remains the same as in the 1→1.5shift mentioned above.

1.5→1 shift

The control timing during this shift mode is shown in FIG. 12d. Thisshift mode is again a single change shift in which the subtransmission 2is changed from its high to its low condition while the maintransmission 3 continues to operate in the first speed range. In thisshift mode, the timing solenoid SL5 continues to be turned off (to theduty cycle of 0%), while the on/off condition of the solenoids SL1 toSL3 are changed to those used during the first speed range upon enteringthe TE interval. The linear solenoid SL6 is energized with a duty cycleor current value shown in FIG. 9c which corresponds to the throttlevalve opening θ and the rotational speed Nt of the output shaft of thesubtransmission 2 during the TE interval also.

2→1.5 shift

The control timing during this shift mode is shown in FIG. 12e. Thisshift mode is a double change shift in which the main transmission 3 ischanged from the second to the first speed range while thesubtransmission 2 is changed from its low to its high condition. In thisshift mode, upon entering the TE interval, the on/off condition of thesolenoids SL1 and SL2 are changed from those associated with the secondspeed range to those associated with the 1.5 speed range (even thoughthose used in the first speed range are used as far as the maintransmission 3 is concerned), but the solenoid SL3 is changed from offto high condition (or from low to high in the subtransmission 2) onlyafter T15S (0.4 sec) has passed or only after the timer T4 (0.4 sec)which is started at step 49 in FIG. 5c has timed out. In other words,upon entering the TE interval, the first speed range is initiallyestablished, and the 1.5 speed range is established at T15S thereafter.The timing solenoid SL5 continues to be turned off (to the duty cycle of0%) until TDL (2 sec) passes upon entering the TE interval, whereupon itis turned on (to the duty cycle of 100%). The linear solenoid SL6 isenergized with the duty cycle or current value shown in FIG. 9c whichcorresponds to the throttle valve opening θ and the rotational speed Ntof the output shaft of the subtransmission 2 during the TE intervalalso.

3→1.5 shift

The control timing during this shift mode is schematically shown in FIG.12f. This shift mode is a double change shift in which the maintransmission 3 is changed from the third to the fourth speed range whilethe subtransmission 2 is changed from its low to its high condition. Thedetail of the control is similar to the control used during 2→1.5 shift.

4→1.5 shift

The control timing during this shift is shown in FIG. 12g. Even thoughthis shift mode is a single change shift in which the main transmission3 is changed from the third to the fourth speed range while thesubtransmission 2 is maintained high, this is achieved by initiallychanging from the fourth to the third speed range (by changing thesubtransmission 2 from its high to its low condition, or equivalentlychanging the solenoid SL3 from its on to its off condition), asillustrated in FIG. 12g. When the TE interval passes, timer T2 (0.2 sec)and timer T2D (0.2+TDL sec where TDL represents a delay time) arestarted (55 to 63 in FIG. 5c). When the timer T2 times out, the 1.5speed range is established (70 to 79 in FIG. 5e). Specifically, thesolenoid SL3 is changed from its off to its on condition, and thesolenoid SL2 from its off to its on condition. In other words, thesubtransmission 2 is changed from its low to its high condition and themain transmission 3 is changed from the third to the first speed range.The timing solenoid SL5 continues to be turned off (or the duty cycle of0%) until the timer T2D times out, whereupon it is turned on (to theduty cycle of 100%) (72 to 76 in FIG. 5e). ATIN represents the outputregister associated with the solenoid SL5. The linear solenoid SL6 isenergized with the duty cycle or a current level shown in FIG. 9c whichcorresponds to the throttle valve opening θ and the rotational speed Ntof the output shaft of the subtransmission 2 during the shift operation.

(12) Steady-state lockup control.

In the lockup control (67) shown in FIG. 5d, when the solenoid SL4 isoff or the lockup, in which the directly coupled clutch 50 is engagedand solenoid SL4 is on, is not activated, a particular reference vehiclespeed data group having the throttle valve opening θ as a parameter isspecified in order to determine the activation of the lockup dependentupon an involved speed range, provided the shift lever is in its Dposition and the brake switch 137 is turned off, meaning that the brakepedal is not depressed. From this data group, particular referencevehicle speed data is specified which corresponds to the currentthrottle valve opening θ, and the current vehicle speed No is examinedif it is equal to or greater than the reference vehicle speed data. Ifthe current vehicle speed No is found to be equal to or greater than thereference vehicle speed data, this means that there is a need toactivate the lockup. Accordingly, the lockup is activated by energizingthe solenoid SL4. However, in order to prevent any shock from resultingfrom the lockup, the duty cycle of the solenoid SL4 is increasedstepwise as shown in FIG. 14a. Specifically, when the need for thelockup is found, a duty cycle Dt1 corresponding to the current throttlevalve opening θ is determined and is written into the output registerALIN. The relationship between the opening θ and the duty cycle Dt1 isgraphically shown in FIG. 14b. Timers TLB (0.4 sec) and TLON (1.0 sec)are started. Subsequently, the same duty cycle is maintained until thetimer TLB times out, provided the lockup condition prevails. When thetimer TLB times out, a duty cycle Dt1 (FIG. 14b) corresponding to thethrottle valve opening θ which then prevails is again calculated, andthe duty cycle is updated to this new calculated value. When the timerTLON times out, the duty cycle is changed to 100%. This completes thelockup activation control for engaging the directly coupled clutch 50.

During the lockup and during the lockup activation control, the currentvehicle speed No is monitored if a condition to terminate the lockupapplies. Specifically, a particular reference vehicle speed data grouphaving the throttle valve opening as a parameter is specified in orderto determine the termination of the lockup dependent upon the particularspeed range. Particular reference vehicle speed data which correspondsto the current throttle valve opening θ is specified among this datagroup, and the current vehicle speed No is examined if it is equal to orless than the reference vehicle speed data. In the event the currentvehicle speed No is equal to or less than the reference vehicle speed,the lockup must be terminated. Accordingly, the solenoid SL4 is turnedoff (or the duty cycle of 0%). At this end, data specifying a duty cycleof 0% is written into the output register ALIN associated with thesolenoid SL4. However, in order to prevent a hunting between theactivation and the termination of the lockup, the determination of thenecessity to terminate the lockup is not executed during an interval of0.5 sec after the timer TAON has timed out or after the completion ofthe lockup activation control.

If the necessity to terminate the lockup is found before the timer TLONtimes out during the lockup activation control, the solenoid SL4 isimmediately turned off (or the duty cycle of 0%). When the need for ashift operation is found before the timer TLB times out, the solenoidSL4 is immediately turned off. When a need for a shift operation isfound after the timer TLB has timed out, but before the timer TLON timesout, the control mentioned under the next paragraph (13) is performed.

(13) Control of energization of the lockup control solenoid SL4 when ashift operation is to occur during the lockup.

Upon entering the TE interval for the control of an up shift, CPU in thecontrol board 130 once decreases the duty cycle of the lockup activationsolenoid SL4 and then increases it stepwise so that the duty cyclereturns to 100% after the TE interval has passed, as shown in FIG. 14c,in order to allow shift shocks to be accommodated for, to a certaindegree, by the torque converter 1. Specifically, upon entering the TEinterval, the duty cycle of the lockup activation solenoid SL4 isdecreased to Dt2 which corresponds to the prevailing throttle valveopening θ, and timer TLS (0.4 sec) is started. FIG. 14d graphicallyshows the relationship between the throttle valve opening θ and the dutycycle Dt2. When the timer TLS times out, a duty cycle Dt2 whichcorresponds to the prevailing throttle valve opening θ is againcalculated (FIG. 14d), and the duty cycle of the solenoid SL4 is updatedto this calculated value. Upon termination of the TE interval, the dutycycle of the solenoid SL4 is returned to 100%.

For a down shift, the solenoid SL4 is immediately turned off (or theduty cycle of 0%) upon entering the TE interval. Subsequently, thelockup is activated according to the paragraph (12).

(14) Lockup control associated with a change in the shift leverposition.

When the brake switch 137 is turned on, meaning that the brake pedal hasbeen depressed, or when the shift lever is changed to one of S, L, N, Ror P position, the lockup is immediately terminated by turning thesolenoid SL4 off.

When the shift lever is in its S position, the lockup is activatedaccording to the lockup activation control mentioned under the paragraph(12), provided No≧700 rpm, the idling switch 139 being closed, meaningan idling condition, and Ne<Nt are simultaneously satisfiled. When theshift lever is changed from S position to a different range, when No<700rpm to prevail or when the idling switch 139 is opened (by opening thethrottle valve), the lockup is immediately terminated by turning thesolenoid SL4 off.

As described, CPU in the control board 130 writes 1 into register LSFwhen the shift lever position as detected by the switch 134 is in the Lrange and LS (1.5 speed range) command switch 131 is closed, indicatingthe presence of such command (7-11-12), and during the shift decision(14), 1.5 speed range is written into register DS as a speed range whichis to be established next, and determines that there is a need for ashift operation (15) when the current speed range as represented by thecontent of the register PS differs from the 1.5 speed range stored inthe register DS, followed by executing the shift operation from thecurrent speed range which may be the first, the second, the third or thefourth speed range to the 1.5 speed range (FIGS. 12a, 12e, 12f or 12g).

Subsequently when the switch 131 is opened, meaning the absence of thecommand for the 1.5 speed range or when the shift lever is changed fromL range to S or D range, the register LSF is cleared (7, 11-13), andduring the shift decision (14), a particular speed range (range X) inthe S or D range which corresponds to the current throttle valve openingθ and the rotational speed No of the wheel driving shaft is determinedand is written into the register DS, thus performing a 1.5→X shift whereX=1, 2, 3 or 4; see FIGS. 12b, 12c or 12d).

Subsequently, as long as the shift lever is in the D range, a shiftoperation occurs only between the first, the second, the third and thefourth speed range. When the shift lever is in the S range, the shiftoperation takes place between the first and the second speed range. Ifthe switch 131 is open, indicating the absence of any command for the1.5 speed range, only the first speed range is established in the Lrange. In these instances, the 1.5 speed range is not established.

As described, in accordance with the invention, the delay time betweenthe termination of a speed range which prevails before a shift operationto the activation of a new speed range to be established is increased bythe timing control means (SL5) in direct proportion to the throttlevalve opening (θ) and in inverse proportion to the rotational speed (No)(136 to 144 in FIG. 8b), whereby the occurrence of shift shocks issuppressed.

In addition, at the commencement of the shift operation, the time delayis determined by the timing control valve (SL5) in accordance with theopening (θ) and the rotation speed (No), and subsequently the delay timeis again updated in accordance with the opening (θ) and the rotationalspeed (No). Hence the delay time can be properly modified in response toany change in the opening (θ) and the rotational speed (No) during theshift time interval (a time interval equal to or less than 0.4 sec fromthe commencement of the shift operation), thus reducing the probabilitythat shift shocks may result.

While a preferred embodiment of the invention has been illustrated anddescribed, it is to be understood that there is not intention to limitthe invention to the precise construction disclosed herein and that theright is reserved to all changes and modifications coming within thescope of the invention as defined in the appended claims.

What is claimed is:
 1. A hydraulic control system for an automatictransmission comprising:a hydraulic circuit for selectively supplying anoil pressure to or selectively removing an oil pressure from brakes andclutches disposed between an output shaft of an engine and a loaddriving shaft; accumulators in the hydraulic circuit and connected tothe brakes and clutches; a pressure control valve for regulating a backpressure on the accumulators; shift control means for determining ifthere is a need to effect a shift operation and for performing the shiftoperation whenever such need is found; an electrically energized timingcontrol valve for providing a time delay between termination of thetransmission of a torque in a speed range which prevails before theshift operation and initiation of the transmission of a torque in a newspeed range; opening detecting means for detecting an opening of athrottle valve of the engine; speed detecting means for detecting arotational speed of an output shaft of the automatic transmission; anddelay control means, operable whenever the shift control means performsa shift operation, to determine an energization level for energizingsaid timing control valve which results in a longer time delay for agreater throttle valve opening and a shorter time delay for a higherrotational speed and to energize the timing control valve in accordancewith such energization level, and subsequently at a given time intervalthereafter, to determine an energization level in accordance with thethrottle valve opening and the rotational speed which again results in alonger delay time for a higher throttle valve opening and a shorterdelay time for a higher rotational speed and to energize the timingcontrol valve in accordance with such energization level.